Basic Principles of Batteries and Electrochemical Cells
Batteries may be divided into two principal types, primary batteries and secondary batteries. Primary batteries may be used once and are then exhausted. Secondary batteries are also often called rechargeable batteries because after use they may be connected to an electricity supply, such as a wall socket, and recharged and used again. In secondary batteries, each charge/discharge process is called a cycle. Secondary batteries eventually reach an end of their usable life, but typically only after many charge/discharge cycles.
Secondary batteries are made up of an electrochemical cell and optionally other materials, such as a casing to protect the cell and wires or other connectors to allow the battery to interface with the outside world. An electrochemical cell includes two electrodes, the positive electrode or cathode and the negative electrode or anode, an insulator separating the electrodes so the battery does not short out, and an electrolyte that chemically connects the electrodes.
In operation the secondary battery exchanges chemical energy and electrical energy. During discharge of the battery, electrons, which have a negative charge, leave the anode and travel through outside electrical conductors, such as wires in a cell phone or computer, to the cathode. In the process of traveling through these outside electrical conductors, the electrons generate an electrical current, which provides electrical energy.
At the same time, in order to keep the electrical charge of the anode and cathode neutral, an ion having a positive charge leaves the anode and enters the electrolyte and a positive ion also leaves the electrolyte and enters the cathode. In order for this ion movement to work, typically the same type of ion leaves the anode and joins the cathode. Additionally, the electrolyte typically also contains this same type of ion. In order to recharge the battery, the same process happens in reverse. By supplying energy to the cell, electrons are induced to leave the cathode and join the anode. At the same time a positive ion, such as Li+, leaves the cathode and enters the electrolyte and a Li+ leaves the electrolyte and joins the anode to keep the overall electrode charge neutral.
In addition to containing an active material that exchanges electrons and ions, anodes and cathodes often contain other materials, such as a metal backing to which a slurry is applied and dried. The slurry often contains the active material as well as a binder to help it adhere to the backing and conductive materials, such as a carbon particles. Once the slurry dries it forms a coating on the metal backing.
Unless additional materials are specified, batteries as described herein include systems that are merely electrochemical cells as well as more complex systems.
Several important criteria for rechargeable batteries include energy density, power density, rate capability, cycle life, cost, and safety. The current lithium-ion battery technology based on insertion compound cathodes and anodes is limited in energy density. This technology also suffers from safety concerns arising from the chemical instability of oxide cathodes under conditions of overcharge and frequently requires the use of expensive transition metals. Accordingly, there is immense interest to develop alternate cathode materials for lithium-ion batteries. Sulfur has been considered as one such alternative cathode material.
Lithium-Sulfur Batteries
Lithium-sulfur (Li—S) batteries are a particular type of rechargeable battery. Unlike most rechargeable batteries in which the ion actually moves into and out of a crystal lattice, the ion in lithium-sulfur batteries reacts with lithium in the anode and with sulfur in the cathode even in the absence of a precise crystal structure. In most Li—S batteries, the anode is lithium metal (Li or Li0). In operation, lithium leaves the metal as lithium ions (Li+) and enters the electrolyte when the battery is discharging. When the battery is recharged, lithium ions (Li+) leave the electrolyte and plate out on the lithium metal anode as lithium metal (Li). At the cathode, during discharge, particles of elemental sulfur (S) react with the lithium ion (Li+) in the electrolyte to form Li2S. When the battery is recharged, lithium ions (Li+) leave the cathode, allowing to revert to elemental sulfur (S).
Sulfur is an attractive cathode candidate as compared to traditional lithium-ion battery cathodes because it offers an order of magnitude higher theoretical capacity (1675 mAh g−1) than the currently employed cathodes (<200 mAh g−1) and operates at a safer voltage range (1.5-2.5 V). This high theoretical capacity is due to the ability of sulfur to accept two electrons (e−) per atom. In addition, sulfur is inexpensive and environmentally benign.
However, the major problem with a sulfur cathode is its poor cycle life. The discharge of sulfur cathodes involves the formation of intermediate polysulfide ions, which dissolve easily in the electrolyte during the charge-discharge process and result in an irreversible loss of active material during cycling. The high-order polysulfides (Li2Sn, 4≦n≦8) produced during the initial stage of the discharge process are soluble in the electrolyte and move toward the lithium metal anode, where they are reduced to lower-order polysulfides. Moreover, solubility of these high-order polysulfides in the liquid electrolytes and nucleation of the insoluble low-order sulfides (i.e., Li2S2 and Li2S) result in poor capacity retention and low Coulombic efficiency. In addition, shuttling of these high-order polysulfides between the cathode and anode during charging, which involves parasitic reactions with the lithium anode and re-oxidation at the cathode, is another challenge. This process results in irreversible capacity loss and causes the build-up of a thick, irreversible Li2S barrier on the electrodes during prolonged cycling, which is electrochemically inaccessible. Overall, the operation of Li—S cells is so dynamic that novel electrodes with optimized compositions and structure are needed to maintain the high capacity of sulfur and overcome the challenges associated with the solubility and shuttling of polysulfides. Thus, in prior Li—S batteries, polysulfides have been treated as an undesirable by-product of useful electrochemical reactions. For example, some prior batteries have sought to trap polysulfide within the cathode and have achieved an actual, reversible utilization of 1.3 e−1 per sulfur atom.
Very early Li—S batteries used a Li/dissolved sulfur polysulfide systems in which a dissolved polysulfide (Li2Sn, n≧8) electrolyte was used as a catholyte with a polytetrafluoroethylene-bonded carbon electrode. These batteries exhibited the ability to utilize 1.6 e− per sulfur atom, but deteriorated after only ten to twenty cycles. A more recently construed similar battery using a porous carbon foam electrode similarly degraded after around twelve cycles. Thus, attempts to use polysulfides within a battery, rather than treat it is undesirable, have not been successful in obtaining a practical rechargeable battery.
In addition to problems with polysulfide formation, sulfur is an insulator with a resistivity of 5×10−30 S cm−1 at 25° C., resulting in a poor electrochemical utilization of the active material and poor rate capacity. Although the addition of conductive carbon to the sulfur material could improve the overall electrode conductivity, the core of the sulfur particles, which have little or no contact with conductive carbon, will still be highly resistive.
Previous attempts to address the conductivity problem have sought to increase the fraction of sulfur in contact with carbon. Several approaches have been pursued, such as forming sulfur-carbon composites with carbon black or nanostructured carbon. For example, a mesoporous carbon framework filled with amorphous sulfur with the addition of polymer has been found to exhibit a high reversible capacity of approximately 1000 mAh g−1 after 20 cycles. However, most traditional methods to synthesize sulfur-carbon composites include processing by a sulfur melting route, resulting in high manufacturing costs due to additional energy consumption. Also, several reports have noted that the sulfur content in the sulfur-carbon composites synthesized by the sulfur melting route is limited to a relatively low value in order to obtain acceptable electrochemical performance, leading to a lower overall capacity of the cathode.
Moreover, synthesizing homogeneous sulfur-carbon composites through conventional heat treatment is complicated. In the conventional synthesis of sulfur-carbon composites, sulfur is first heated above its melting temperature, and the liquid sulfur is then diffused to the surface or into the pores of carbon substrates to form the sulfur-carbon composite. A subsequent high-temperature heating step is then required to remove the superfluous sulfur on the surface of the composites, leading to a waste of some sulfur. Thus, the conventional synthesis by the sulfur melting route may not be scaled-up in a practical manner to obtain a uniform industry-level sulfur-carbon composite.
As another alternative, a sulfur deposition method to synthesize a core-shell carbon/sulfur material for lithium-sulfur batteries has been recently reported. Although this process exhibited acceptable cyclability and rate capability, the sulfur deposition process is very sensitive and must be carefully controlled during synthesis. Otherwise, a composite with poor electrochemical performance is produced.
A more recent methodology of forming a sulfur-carbon composite overcomes many of these problems, but still experiences difficulties with polysulfides.
Therefore, there remains a need for a rechargeable Li—S battery with high reversibility and good cycle life that is able to constructively use polysulfides.